1. Technical Field
The present invention relates to a momentum transfer type turbo vacuum pump that discharges gas, and more specifically to a turbo vacuum pump suitable for applications in which a large flow of gas is discharged.
2. Related Art
As shown in FIG. 13, a turbo vacuum pump 101 according to the prior art includes a discharge part 150, a motion control part 151, a rotating shaft 121, and a casing 153 that houses the discharge part 150, the motion control part 151, and the rotating shaft 121. The rotating shaft 121 is arranged vertically from top to bottom.
The casing 153 has an upper housing 123, a lower housing 137 arranged below the upper housing 123, and a sub-casing 140 arranged between the upper housing 123 and the lower housing 137. The upper housing 123 has a suction nozzle 123A, and the sub-casing 140 has a discharge nozzle 123B formed on its side face. The upper housing 123 houses the discharge part 150, and the portion of the rotating shaft 121 on the discharge part 150 side. A suction opening 155A is formed in the suction nozzle 123A, and a discharge opening 155B is formed in the discharge nozzle 123B. The suction nozzle 123A sucks gas from the suction opening 155A, and the discharge nozzle 123B discharges the sucked gas from the discharge opening 155B.
The discharge part 150 includes a plurality of (five) stages of stationary impellers 171,128, a turbine impeller part 173 having a plurality of (three) stages of turbine impellers 170, and a plurality of (three) stages of centrifugal impellers (centrifugal drag impellers) 124. The stationary impellers 171 are formed in three stages, and arranged immediately downstream of the respective turbine impellers 170. The stationary impellers 128 are formed in two stages, and arranged immediately downstream of the first and second stages of the centrifugal impellers 124. Gas exiting the turbine impeller 170 of the last stage is sucked into the centrifugal impeller 124 of the first stage.
A hollow part 112 is formed in a boss part 174 of the turbine impeller part 173, and a through hole 158 is formed at a bottom part 112B of the hollow part 112. A screw hole 118 is formed in a suction-part-side end face 115 in the upper portion of the rotating shaft 121. The turbine impeller part 173 is mounted to the suction-part-side end face 115, being fixed with a hexagonal bolt 178. That is, the hexagonal bolt 178 is inserted into the through hole 158 of the turbine impeller part 173, and is further inserted into the screw hole 118 of the rotating shaft 121, thus fixing the turbine impeller part 173 to the suction-part-side end face 115 of the rotating shaft 121 (for example, Patent Document 1: JP-A-2007-192076).
However, in the turbo vacuum pump 101 mentioned above, since the turbine impeller part 173 having the turbine impellers 170 is mounted to the suction-part-side end face 115 of the rotating shaft 121, the natural frequency of the rotor as a whole including the rotating shaft 121, the turbine impeller part 173, and the centrifugal impellers 124 decreases.
Accordingly, it is an object of the present invention to provide a turbo vacuum pump that makes it possible to enhance the natural frequency of the rotor as a whole to perform stable high-speed rotation, and enables high-speed rotation to thereby achieve a reduction in size and weight.